In water-cooled nuclear reactors, the reactor core in which the fission chain is sustained generally contains a multiplicity of fuel element assemblies, also known as fuel assemblies, that are identical in mechanical construction and mechanically interchangeable in any core fuel assembly location. Each fuel assembly is designed to maintain its structural integrity during reactor heatup, cool-down, shut-down and power operations including the most adverse set of operating conditions expected throughout its lifetime. Design considerations for reactor operation include the combined effects of flow induced vibration, temperature gradients, and seismic disturbances under both steady state and transient conditions.
Each fuel assembly typically contains a plurality of thin elongated fuel elements, a number of spacer grid assemblies, guide tubes, an instrumentation tube, and end fittings.
The fuel elements used in current applications are known as fuel rods and comprise cylindrical UO.sub.2 fuel pellets stacked end to end within a thin walled tube (cladding), often having spring loaded plenums at each end of the tube, that is hermetically sealed with end caps or plugs. The fuel rod cladding is made from a material, such as a zirconium alloy, which has good neutron economy, i.e. a low capture cross section. Depending upon the position of a fuel assembly within the core, a number of elongated cylindrical guide tubes, arranged in parallel with fuel rods, are used variously to provide continuous sheath guidance for axially translatable control rods, axial power shaping rods, burnable poison rods, or orifice rods. Sufficient internal clearance is provided to permit coolant flow through the guide tubes to limit the operating temperatures of the absorber materials which may be inserted therein, and to permit rod insertion and withdrawal motions therewithin during all phases of reactor operation. The guide tubes have a larger cross-section or diameter than the fuel rods.
Each fuel rod transfers nuclear fission generated heat to circulating coolant water, circulating through parallel flow passages or coolant channels between the adjacent parallelpiped fuel rods, guide tubes and the instrumentation tube. The coolant channels are associated with an effective flow area transverse to the channel length. The various types of coolant channels are alternatively defined by the flow area between adjacent fuel rods (known as unit channels), by the flow area between a guide tube or instrument tube and adjacent fuel rods (known as guide tube channels), or by the flow area between fuel rods and an adjacent flow barrier such as the thermal shield of the reactor. The flow area of a guide tube channel is smaller than the flow area of a unit channel due to the larger cross-section of the guide tubes.
In each fuel assembly, fuel rods, guide tubes and instrumentation tube are typically supported in a square array at intervals along their lengths by spacer assemblies that maintain the lateral spacing between these components in an open-lattice arrangement. Each spacer assembly is generally composed of a multiplicity of slotted rectangular grid plates arranged to intersect and interlock in an egg-crate fashion to form cells through which the fuel rods and guide tubes extend in a parallel array. Illustratively, the grid plates may be of the type, such as described in U.S. Pat. No. 3,665,586 by F. S. Jabsen and assigned to The Babcock & Wilcox Company, which have indentations that laterally extend essentially perpendicular to the longitudinal axes of the fuel rods into those cells which have fuel rods for engagement and support of the fuel rods. These spacer grids typically accommodate and support the larger control rod guide tubes and the instrument tube in cells not having such indentations. Despite the difference in the diameter of the fuel rods relative to that of the guide tubes or instrumentation tube, all of these parallelpiped components are arrayed in a uniform square pitch, that is have equal center to center distance, within the fuel assembly.
The spacer assemblies maintain a necessarily precise spacing between fuel rods in order to avoid neutron flux peaks and regions of abnormally high temperature (hot spots) where burnout, i.e. severe local damage to the fuel rods, could occur. The spacer assemblies assure the mechanical stability that is essential to preclude the distortions which may be otherwise caused by flow induced vibrations, pressure differences, and thermal stress.
Coolant typically flows upwardly through the flow channels parallel to the surrounding members. Since the flow areas of the channels differ, the flow rate is not the same in each type of flow channel.
The design of the reactor core is limited by the heat transfer rate from the fuel to the coolant. The limiting or "critical" heat flux is defined by the onset of the departure from nucleate boiling (DNB). This condition marks the transition into an area of low heat transfer coefficient and a very high fuel element surface temperature which can eventually lead to burnout. DNB can occur if the fuel element heat flux is too great for a given coolant flow. Reactor core design criteria, therefore, are partly based on the establishment of a maximum permissible heat flux in the so-called "hot channel" which is a fraction of the calculated burnout flux. Safety margins between the maximum permissible heat flux and the critical heat flux, characterized as "minimum DNB ratios," are set for the various flow channel types in order to provide an adequate margin of safety under all conditions.
Experimental studies indicate that critical heat flux values in fuel element assemblies are generally lower in the smaller guide tube channels than in unit channels. Thus, the guide tube channels are, in the sense, a limiting factor in reactor operation, particularly within the "gap" defining the closest spacing between a guide tube and adjacent fuel rod.